Typical conversion circuits or mixers multiply an input signal with a local oscillator LO signal. The multiplication can for example be performed by switching the currents to obtain a low noise and high linearity result.
Typical mixers like switching MOS mixer and a Gilbert cell mixer are depicted in FIG. 1. The switching MOS mixer comprises four MOS transistors TA, TB, TC, and TD. The transistors TA and TB are coupled to a first resistor RS and the transistors TC and TD are coupled to a second resistors RS. The two resistors RS are coupled to an RF input of the mixer. Furthermore, an operational amplifier OP is coupled between the transistors TA, TB, TC and TD and an IF output. The transistors TA and the transistor TC are coupled to a first input of the operational amplifier OP while the transistors TB and TD are coupled to the second input of the operational amplifier. The gates of the transistor TA and TD are coupled together and the gates of the transistors TB and TC are coupled together. The local oscillator signal LO is applied as input to the gate terminals of the transistors. The mixing of the RF input signal with the LO signal is performed by switching the transistors TA, TB, TC, and TD.
FIG. 2 shows a basic representation of a square wave multiplication of an up-conversion mixer according to the prior art. The above-mentioned multiplication corresponds to a multiplication of the input signal with a local oscillator LO signal having a square wave (as depicted in FIG. 2), i.e. the harmonics of the square wave are also included in the multiplication. Therefore, several unwanted conversion or multiplication components may be present at the output of the mixer. Accordingly, for a down-conversion receiver with a basic balance switching mixer the required channel at an IF distance from the L0 frequency is down converted. However, additionally RF signals at IF distance from 3·LO, 5·LO, 7·LO, etc. are also converted.
Narrow-band receivers are designed with a RF selectivity which are arranged before the mixer for removing any signals which would otherwise be down-converted by the LO harmonics. On the other hand, wide-band receivers will require tracking filters or harmonic rejection mixers. A harmonic rejection mixer is used to suppress the down-conversion or up-conversion with some LO harmonics that would otherwise occur. Ideally, this could be performed by a linear multiplication as shown for an up-conversion mixer in FIG. 3. However, in general, a bad noise and a bad linearity performance is present. In principle, a harmonic rejection mixer can by used for an up-conversion (the output frequency is higher than the input frequency)) and for a down conversion (the output frequency is lower than the input frequency). In an up-conversion the harmonic rejection mixer can be used to prevent multiples of the harmonics.
In “A 1.75 GHz Highly Integrated Narrow-Band CMOS Transmitter With Harmonic-Rejection Mixers”, IEEE Journ. Solid State Circuits, Vol. 36, No 12, December 2001, an alternative solution is shown, where the effective shape of the LO signal can be changed by combining a number of switching multipliers and time shifting the LO signals applied to each of the multipliers or their respective transistors.
US 2004/005869 discloses a further harmonic rejection mixer with a number of Gilbert cells.
US 2005/0059376 describes a method for frequency translation with harmonic suppression using mixer stages.
However, harmonic rejection mixers according to the prior art suffer from a bad noise performance as compared to basic switching mixers. Moreover, the number of transistors which have to be switched in harmonic rejection mixer is quite high. This also leads to a high local oscillator LO driver power (as the local oscillator circuit must be able to drive all transistors) and a large silicon area. Moreover, due to cross talk, the layout of these transistors is complex. Furthermore, calibration loops which are incorporated to improve the limited rejection during the conversion of the LO harmonics (due to a mismatch of capacitive and/or resistive parasitics) are highly complicated as several parameters have to be aligned.